You are fine both legally and ethically. If kobo had a different policy, and you shared your books with your mom -- you'd be doing something illegal. Unethical? Well, lets just say that _I_ see a big difference between sharing a book within your family and putting it up on the net where thousands of folks can get it without paying.
For example -- I know it's illegal to get a copyrighted book from the "darknet". However, if I check out a hard back book from the library, and then get the ebook from the darknet so that I could read the book on my iPad, then delete the file when I take the hard back back to the library. I don't think I've been unethical, but that's a matter of personal opinion.
Oh, and I don't mean ME, I mean 'my friend' -- as I surely would never do anything illegal or unethical
Lee